The long-term objectives of this project are to develop a high-performance, cost-effective clinical prototype of a unique breast imaging system, HABIS, that uses non-ionizing ultrasound for examination of the breast and to demonstrate the medical value and clinical efficacy of the system through trials of the system in medical-center and outpatient imaging-center settings. The immediate objectives in Phase I of this SBIR application are to confirm that HABIS meets acoustic and electrical safety requirements as well as expectations for resolution, and to accumulate a portfolio of initial human subject images that will aid the planning of more extensive human trials in a later SBIR Phase-II project. The imaging system is designed to overcome x-ray mammography limitations of poor resolution of contrast in dense breasts (i.e., breasts with high x-ray attenuation), discomfort and anatomical deformation induced by compression, and poor imaging of breasts with implants. The system consists of a custom data-acquisition apparatus, a high-performance computer network, and novel software that controls the collection of data and forms volumetric images having isotropic high resolution (100 m). These images are reconstructions of sound speed and attenuation slope based on inverse scattering as well as aberration-corrected b-scans. Unique features of the system are: collection of scattering data from the entire breast volume in about two seconds without any moving parts to minimize image degradation from motion, extrapolation of measured scattering to obtain virtual measurements in the opposite hemisphere where actual measurements are impossible to obtain directly, and simultaneous independent reconstruction of subvolumes to speed the reconstruction process. Phase I has four specific aims: 1) Conduct acoustic and electrical tests to confirm that HABIS is safe for patient use and secure approval for human imaging studies. 2) Quantify HABIS point and contrast resolution and confirm that resolution meets calculated expectations by using images of contrast and resolution phantoms. 3) Develop image acquisition and review protocols suitable for use by technicians and clinicians, and optimize imaging algorithms to ensure efficient validation of data acquisition and clinical review of quantitative and b-scan images. 4) Assemble a portfolio of HABIS images acquired from 10-20 human subjects with normal as well as abnormal mammographic results that will be used to guide the development of a plan for extensive human trials in an SBIR Phase-II project. The major results of the Phase-I aims would be proof that HABIS complies with standards for patient safety, approval for human studies by an institutional review board, quantified point and contrast resolution of HABIS, optimized HABIS software suitable for patient imaging to be conducted by clinical collaborators, and human subject images that will provide preliminary evidence of the medical value of HABIS and indicate questions to be addressed in SBIR Phase-II human trials. Achievement of the long-term goals of this project would lay the foundation for additional studies that could change the present clinical paradigm of imaging for breast-cancer detection, diagnosis, and monitoring of response to treatment.